The adsorption of oxygen on NiAl(001) at 300 K and 20 K

The adsorption of oxygen on NiAl at 300 K and 20 K has been studied by means of high resolution electron energy loss spectroscopy (EELS), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). The adsorption of oxygen at 300 K leads to formation of a thin film of amorphous aluminum oxide (a-Al₂O₃). After oxygen adsorption at 300 K and annealing of the oxygen-saturated surface to 1200 K, a well-ordered thin θ-Al₂O₃ film is formed. After annealing of the oxygen-saturated surface to 1400 K, an α-like Al₂O₃ oxide is grown. Even at 20 K oxidation of the NiAl(001) surface is found. However, at higher exposure (≥ 5 L) oxygen physisorbs and the EEL spectrum exhibits characteristics of resonance scattering. The resonance energy for physisorbed oxygen on the amorphous Al oxide layers formed at 20 K has a value of 7.0 eV. The frequency of O-O stretching vibration for physisorbed O₂ is 193 meV. The line shape of this mode is asymmetric with a pronounced tail on the high energy side. The asymmetric line shape corresponds to multiple losses due to the overlap of the O-O stretching mode with low frequency modes. After physisorption of oxygen at 20 K on the α-like Al oxide layers, the resonance energy is shifted to 9.0 eV, a value close to that of oxygen in the gas phase.